Honeycomb structure and seal material

ABSTRACT

A honeycomb structure is formed by adhering a plurality of honeycomb units provided with cells to each other through a layer of a seal material layer, in which the layer of the seal material comprises an inorganic binder and oxide particles having a particle size of about 0.01 to about 100 μm.

FIELD OF THE INVENTION

This invention relates to a honeycomb structure and a seal material usedin the honeycomb structure.

BACKGROUND ART

As the honeycomb structure used in the purification apparatus for theexhaust gas of the internal engine, there are a one-piece honeycombstructure of cordierite, an assembly type honeycomb structure formed byintegrally adhering a plurality of silicon carbide honeycomb unitsthrough a seal material (adhesive material), and so on. As the sealmaterial is recommended a material constituted with ceramic particles ofa carbide or a nitride having a high thermal conductivity such assilicon carbide, aluminum nitride or the like (see, for example,JP-A08-28246 and JP-A2004-130176). As an example of a refractoryadhesive showing a plasticity are known an inorganic aggregate,inorganic fibers, an inorganic binder such as colloidal silica and/orcolloidal alumina, and a fibrous refractory adiabatic compositioncompounded with an aqueous mixed solution of polyvinyl alcohol andmethyl cellulose as an organic binder (see, for example, JP-A64-42373).

The contents of JP-A08-28246, JP-A2004-130176 and JP-A64-42373 areincorporated herein by reference in their entirety.

DISCLOSURE OF THE INVENTION

When an embodiment of the invention is a honeycomb structure of anassembly type of joining a plurality of honeycomb units, the inventionproposes a honeycomb structure characterized by a seal material layerused for adhering the mutual honeycomb units with each other and a sealmaterial layer used for sealing an outer peripheral portion of ahoneycomb block formed by adhering the plurality of the honeycomb unitsas well as the seal material therefor.

That is, an embodiment of the honeycomb structure of the invention is ahoneycomb structure formed by adhering a plurality of honeycomb unitsprovided with cells to each other through a layer of a seal materiallayer, in which the layer of the seal material comprises an inorganicbinder and oxide particles having a particle size of about 0.01 to about100 μm.

In the honeycomb structure according to the invention, it is desirablethat another seal material layer is disposed on an outermost peripheralportion of a honeycomb block formed by joining the plurality of thehoneycomb units so as to surround the surface, and that the oxideparticles in the seal material layer comprise particles of at least oneselected from alumina, zirconia, titania and silica, and that theparticle size of the oxide particles is about 0.1 to about 75 μm.

Also, the honeycomb structure according to the invention is desirablethat the honeycomb unit is an oxide ceramic, that the surface of thehoneycomb unit has an oxide layer, that the oxide layer has a thicknessof about 1 nm to about 1 μm, that the surface of the honeycomb unitcontacting with the seal material layer has a roughness Ra of about 1.0to about 30 μm, that the honeycomb unit is plugged at either one of bothopening ends of the cells with a plugging material, that a catalyst issupported on a wall surface of the cell in the honeycomb unit, and thatit is used as an apparatus for the purification of an exhaust gas in anautomobile.

Furthermore, an embodiment of the seal material according to theinvention is a seal material comprising oxide particles having aparticle size of about 0.01 to about 100 μm and an inorganic binder. Asthe oxide particle comprise desirably used one or more particlesselected from alumina, zirconia, titania and silica.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a honeycomb structure (honeycomb block) 10according to the invention;

FIG. 2 is a schematic view of a honeycomb unit 20 according to theinvention;

FIG. 3 is a view illustrating a state of mounting a filter for thepurification of an exhaust gas constituted with the honeycomb structureaccording to the invention onto an apparatus for the purification of theexhaust gas in an automobile;

FIG. 4 is a graph showing a relation between an alumina particle size ofa seal material and a pushing load when a surface roughness (Ra) of ahoneycomb unit is changed in a silicon carbide honeycomb structure;

FIG. 5 is a graph showing a relation between surface roughness (Ra) of ahoneycomb unit and a pushing load when an alumina particle size of aseal material is changed in a silicon carbide honeycomb structure;

FIG. 6 is a graph showing a relation between an alumina particle size(Ra) of a seal material and a pushing load when an oxide layer thicknessof a honeycomb unit is changed in a silicon carbide structure;

FIG. 7 is a graph showing a relation between an oxide layer thickness ofa honeycomb unit and a pushing load when an alumina particle size of aseal material is changed in a silicon carbide structure;

FIG. 8 is a graph showing a relation between a zirconia particle size ofa seal material and a pushing load when a surface roughness (Ra) of ahoneycomb unit is changed in a fiber-reinforced alumina honeycombstructure; and

FIG. 9 is a graph showing a relation between a surface roughness (Ra) ofa honeycomb unit and a pushing load when a zirconia particle size of aseal material is changed in a fiber-reinforced alumina honeycombstructure.

BEST MODE FOR CARRYING OUT THE INVENTION

As to the seal material used in the honeycomb structure, the inventorshave made various experiments on a method of efficiently applying theseal material, and a heat resistance, durability and the like after theadhesion with the seal material. From this experimental results, it hasbeen found that when the oxide ceramics are used as the seal material,the productivity is high and the sealing property, adhesion strength andresistance to thermal shock are improved. Furthermore, it has been foundthat the above properties of the seal material are influenced by theparticle size in case of using the oxide ceramics.

That is, the honeycomb structure of the invention is particularlycharacterized by the seal material. It has been found that when the sealmaterial mainly contains the oxide ceramic and has a particle size ofabout 0.01 to about 100 μm, preferably about 0.1 to about 75 μm, theproperties as the honeycomb structure such as sealing property, andadhesion strength and durability of unit are considerably improved.

Although the reason is not clear, it has been understood by theinventors as follows. That is, the honeycomb unit being oneconstitutional element in the honeycomb structure of the invention isporous and many opening holes are existent on the surface thereof.Therefore, the inorganic particles as a constitutional component of theseal material become at a state of penetrating into the inside of thehoneycomb unit through the opening holes between the mutually adjoininghoneycomb units. Since the honeycomb units are adhered at this state, itis considered that they are strongly bonded to each other and the highsealing property is obtained.

As the reason producing such action and effect, the inventors think thatthe oxide particles have OH group at their surfaces and chemically bondsto the inorganic binder in the seal material. In the seal material,therefore, when the particle size of the oxide particles is extremelylarge, the contact area with the inorganic binder becomes less and thechemical bonding is weak, while when the particle size of the oxideparticles is extremely small, the bonding between the oxide particle andthe inorganic binder is obstructed to bring about the lowering of theadhesion strength and hence the lowering of the adhesion strengthbetween the honeycomb units. In other words, it is considered that theoxide particles are large in the bonding action with the inorganicbinder as compared with the carbide or the like but the effect isaffected by the size of the oxide particles.

In the invention, it is desirable that the oxide particles constitutingthe seal material are crystalline. Because, when amorphous oxideparticles form a main phase of the seal material, the whole of the sealmaterial becomes at an amorphous ceramic state and these particles areonly existent in the inorganic binder and hence the strength, corrosionresistance and heat resistance as the seal material lower. In thisconnection, the strength, corrosion resistance and heat resistance ofthe seal material can be increased by adding crystalline oxideparticles. Moreover, the judgment of crystalline oxide or amorphousoxide is conducted by an X-ray diffraction, a Fourier transform infraredspectrometer (FT-IR) or the like.

In the invention, the honeycomb unit contacting with the seal materiallayer is preferable to use an oxide ceramic or various ceramics havingan oxide layer at the surface. Because, the oxide ceramic has OH groupat its surface as previously mentioned and is liable to be chemicallybonded to the inorganic binder in the seal material. Even in the case ofnon-oxide ceramic, if the oxide layer is existent on the surface, the OHgroup is existent on the surface of the oxide layer, so that thechemical bonding to the inorganic binder is easily caused.

Also, it is preferable in the invention that the surface roughness Ra ofthe honeycomb unit contacting with the seal material layer (according toJIS-B-0601 (2001)) is adjusted to about 1 to about 30 μm. When thesurface roughness is about 1 μm or more, the contact area between thehoneycomb unit and the seal material layer becomes large and hence theadhesion strength between the honeycomb units does not lower, while whenit is about 30 μm or less, a gap is hardly generated in the concaveportion of the irregularity to bring about no lowering of the adhesionstrength between the honeycomb units.

Moreover, the surface of the honeycomb unit contacting with the sealmaterial layer means a surface of an outermost peripheral portion (outerwall) of a honeycomb block formed by combining the honeycomb units inaddition to the outer peripheral surface of the honeycomb unit itself.

The contents of JIS-B-0601 (2001) are incorporated herein by referencein their entirety.

In the seal material according to the invention is added the inorganicbinder. Particularly, the oxide-based inorganic binder, for example, anoxide sol such as alumina sol, silica sol, colloidal silica or the like,and water glass, colloid of oxide and so on can be used.

Also, the oxide particles are included in the seal material. Forexample, particles of oxide such as alumina, silica, cordierite,mullite, zirconia, aluminum titanate or the like can be used. Thus, theuse of the oxide particles as a main component of the seal material isexcellent in the heat insulating property because of low thermalconductivity and can maintain the temperature inside the honeycomb unitat a high level and hence the activity of the oxide catalyst foroxidizing and removing particulates can be increased.

Furthermore, the seal material may be added with inorganic fibers or thelike in addition to the above inorganic binder and oxide particles. Inthis case, the inorganic fibers are fixed at their cross points by theinorganic binder and the oxide particles are inserted and dispersed intogaps of three-dimensional network structure produced by entangling theseinorganic fibers and such a state is strongly maintained through thepresence of the inorganic binder.

As the inorganic fibers can be used whiskers or the like of oxide,nitride, carbide or the like. For example, alumina fibers, silicafibers, silica-alumina fibers and the like can be used as theoxide-based inorganic fiber, and silicon nitride fibers, titaniumnitride fibers and the like can be used as the nitride-based inorganicfiber, and silicon carbide fibers and the like can be used as thecarbide-based inorganic fiber.

The seal material having the above construction shows an elasticity andhas a high adhesion strength. Particularly, when the oxide particles,oxide inorganic binder and oxide fibers are used as the component of theseal material, there is a merit that cracks resulted from the differenceof thermal expansion coefficient hardly occurs.

Moreover, JP-A64-42373 discloses a heat-resistant composition forthermal equipment comprising inorganic fibers, oxide particles,inorganic binder and organic binder, but recommends the use of oxideparticles having a larger particle size for improving the plasticity,which is different from the particle size defined in the invention.

The honeycomb structure of the invention will be described withreference to the drawings below.

FIG. 1 is a perspective view schematically showing an embodiment of thehoneycomb structure, and FIG. 2( a) is a perspective view schematicallyshowing an embodiment of the honeycomb unit constituting the honeycombstructure 10 shown in FIG. 1 and FIG. 2( b) is a section view thereof.This honeycomb unit 20 has a plurality of cells (through-holes) 21extending from a near side toward a far side, and these cells 21 have ahoneycomb structure of arranging side by side through cell walls 23.Also, end opening portions may be sealed to form a checkered pattern byplugging material 22 for the purpose of purifying the particulates, ifnecessary.

The honeycomb structure 10 is a honeycomb block 15 formed by combining aplurality of honeycomb units 20 in parallel through seal material layers(adhesive layers) 11 and binding them. Further, it is desirable that aseal material layer (coating layer) 12 is disposed on an outermostperiphery of the honeycomb block 15 for causing no leakage of theexhaust gas from a gap to a casing and reinforcing the honeycomb block.The combination of the honeycomb units is called as an assembly typehoneycomb structure. The assembly type honeycomb structure can enhancethe resistance to thermal shock as a whole and the strength tovibrations using the above seal material layers 11, 12 even if themechanical strength, resistance to thermal shock and the like of theindividual honeycomb units are low.

The reason why the strength to thermal shock and vibrations in thehoneycomb structure having the above construction becomes high isconsidered due to the fact that even if the temperature distribution isproduced in the honeycomb structure by the rapid temperature change orthe like, the temperature difference among these honeycomb units issmall and the thermal shock and vibrations are absorbed by the sealmaterial layers. Further, even if cracks are produced in the honeycombunits 20 through thermal stress or the like, the seal material layer hasan action of preventing the propagation of the cracks into the whole ofthe honeycomb structure. Moreover, the seal material layer plays a roleas a protection layer for the honeycomb structure and serves to maintainthe shape of the honeycomb structure over a long time of period toimprove the durability.

The honeycomb unit is preferable to render into an easily joining shape.For example, a section perpendicular to a longitudinal direction of thecell (hereinafter referred to as “unit section”) is desirable to besquare, rectangular or hexagonal and may be a fan-like form.

Also, the honeycomb unit is preferable to have a unit sectional area ofabout 5 to about 50 cm². When the unit sectional area is about 5 cm² ormore, the pressure loss hardly increases. While, when the unit sectionalarea is about 50 cm² or less, the thermal stress produced in thehoneycomb structure can be easily dispersed and cracks are hardlygenerated at a time of applying the thermal stress. In order to make theabove action and effect more remarkable, the unit sectional area ispreferable to be about 6 to about 40 cm², more preferably about 8 toabout 30 cm².

The honeycomb block formed by combining the plurality of the honeycombunits having the above construction, i.e. the honeycomb structure ispreferable to be rendered into a shape such as cylindrical, rectangularpillar, cylindroid or the like.

As a main material (skeleton component) of the honeycomb unit can beused inorganic particles, fibers or whisker of a nitride ceramic such asaluminum nitride, silicon nitride, boron nitride, titanium nitride orthe like; a carbide ceramic such as silicon carbide, zirconium carbide,titanium carbide, tantalum carbide, tungsten carbide or the like; or anoxide ceramic such as alumina, zirconia, cordierite, mullite, aluminumtitanate or the like. Among them, it is preferable to use the siliconcarbide ceramic having excellent heat resistance and mechanicalproperties and a large thermal conductivity. Particularly, thesilicon-containing ceramic compounded with metallic silicon, ceramicsjoined with silicon or silicate compound and the like are preferable.Moreover, the silicon carbide ceramic is not only constituted with onlysilicon carbide, but also includes one composed mainly of siliconcarbide and joined with a metal or a crystalline or amorphous compound.

In the invention, the honeycomb unit comprising essentially of the abovenon-oxide ceramic is desirable to have an oxide layer (possibly about 1nm-about 1 μm) at its surface in addition to the case that at leastsurface is the oxide ceramic. The reason why the oxide layer is formedon the surface is due to the fact that the oxide layer is chemicallybonded to the inorganic binder through OH group in the layer and at thesame time the oxide particles in the seal material layer is chemicallybonded to the inorganic binder as mentioned above and hence thehoneycomb units are strongly adhered through the seal material layer.

Moreover, when the oxide layer has a thickness of about 1 nm to about 1μm, the oxide in the oxide layer is well and chemically bonded to theinorganic binder. However, it is guessed that when the thickness of thislayer is about 1 nm or more, the chemical bond becomes not weak, whilewhen it is about 1 μm or less, the difference of the thermalconductivity becomes not large and cracks are hardly produced.

The material constituting the honeycomb unit may be “multi-mixed typehoneycomb unit” comprising the above skeleton material (main material)and a sub-component (material).

In the multi-mixed type honeycomb unit, it is preferable that the mainmaterial includes at least inorganic ceramic and inorganic binder andthe sub-component includes at least inorganic material (reinforcingmaterial). In such a honeycomb unit, the inorganic ceramic particles canbe bonded through the inorganic binder to provide a strength stablymaintaining the honeycomb form.

In case that the inorganic material as the main material and theinorganic material as the sub-component are different from each other,there can be selected from a case using different components, a casethat the shapes (e.g. particle size, aspect ration or the like) aredifferent even if the same component is used or the properties (e.g.different crystal forms, different melting points or the like). Such amulti-mixed type honeycomb unit is effective to enhance the strength ofthe honeycomb structure.

As the inorganic material of the sub-component can be used one or moreceramic particles selected from silicon carbide, silicon nitride,alumina, silica, zirconia, titania, ceria and mullite. Also, when theinorganic fibers are used as the sub-component, there can be used one ormore inorganic fibers selected from alumina fibers, silica fibers,silicon carbide fibers, silica-alumina fibers, glass fibers, potassiumtitanate fibers and aluminum sulfate fibers. Further, in case of thewhisker, there can be used one or more whiskers selected from alumina,silica, zirconia, titania, ceria and mullite.

The reason why the inorganic binder is used in the production of themulti-mixed type honeycomb unit is due to the fact that it is effectiveto provide the sufficient strength even if the firing temperature of thehoneycomb unit is made low. As the inorganic binder can be usedinorganic sol, clay binder and the like. Among them, as the inorganicsol can be used one or more inorganic sols selected from alumina sol,silica sol, titania sol, water glass and the like. As the clay bindercan be used one or more selected from white clay, kaolin,montmorillonite, composite chain structure type clay (zeolite,attapulgite) and the like.

The honeycomb structure of the invention can be used as a filter for thepurification of the exhaust gas in order to remove particulates includedin the exhaust gas. In this case, the honeycomb unit is a porous memberhaving preferably a porosity of about 20 to about 80%, more preferably aporosity of about 50 to about 70%. When the porosity of the honeycombunit is about 20% or more, the pressure loss of the filter becomes nothigher. While, when the porosity is about 80% or less, the strength ofthe honeycomb structure does not lower and the breakage hardly occurs.Moreover, if the catalyst is provided onto the cell wall of thehoneycomb unit, the pressure loss may be easily increased, so that theporosity is desirable to be about 50 to about 70%. The porosity can bemeasured, for example, by a conventionally known method such as amercury pressure method, an Archimedes method and scanning type electronmicroscope (SEM).

When the honeycomb structure is used as a filter for the purification ofthe exhaust gas for catching particulates included in the exhaust gas topurify the exhaust gas, it is preferable to use a ceramic member havingan average pore size of about 5 to about 100 μm as a honeycomb unit.When the average pore size is about 5 μm or more, the pressure loss ofthe filter to the exhaust gas becomes not higher, while when the averagepore size is about 100 μm or less, the particulates hardly pass throughthe pores and the catching efficiency hardly lowers.

The seal material layer may be a mixture of oxide particles andinorganic binder, a mixture of oxide particles, inorganic fibers andinorganic binder, a mixture of oxide particles, inorganic particles andinorganic binder, a mixture of oxide particles, inorganic fibers,inorganic particles and inorganic binder or a mixture of the abovemixture added with an organic binder.

As the oxide particle may be mentioned oxide ceramic powder, fibers,whisker or the like of alumina, silica, titania, zirconia, cordierite,mullite, aluminum titanate or the like. They may be used alone or in acombination of two or more.

As the inorganic binder may be mentioned silica sol, alumina sol, waterglass and the like. They may be used alone or in a combination of two ormore. Among the above inorganic binders, the use of silica sol isdesirable.

As the inorganic fiber may be mentioned ceramic fibers ofsilica-alumina, mullite, alumina, silica or the like. They may be usedalone or in a combination of two or more. Among the above inorganicfibers, silica-alumina fibers are desirable.

As the inorganic particle may be mentioned carbide ceramic, nitrideceramic and the like. Particularly, inorganic powder or whisker ofsilicon carbide, silicon nitride, boron nitride or the like may bementioned. They may be used alone or in a combination of two or more.Among the above inorganic particles, the use of silicon carbide havingan excellent thermal conductivity is desirable.

As the organic binder are mentioned one or more organic binders selectedfrom polyvinyl alcohol, methyl cellulose, ethyl cellulose, carboxymethylcellulose and the like.

The honeycomb structure having the above construction according to theinvention can be used in the filter for the purification of the exhaustgas. FIG. 3 shows an example of mounting the filter for the purificationof the exhaust gas made from the honeycomb structure according to theinvention onto an exhaust gas purifying apparatus for an automobile. Asshown in this figure, the apparatus 30 for the purification of theexhaust gas comprises a filter F for the purification of the exhaust gasmade of the honeycomb structure of the invention, a casing 31 coveringthe outside of the filter F for the purification of the exhaust gas, anair-tightness support member 32 interposed between the filter F for thepurification of the exhaust gas and the casing 31, and, if necessary, aheating means (not shown) arranged at a side of introducing the exhaustgas, in which an end portion of the casing 31 at the inlet side of theexhaust gas is connected to an inlet pipe 33 connected to an internalcombustion such as an engine or the like and the other end portion ofthe casing 31 is connected to an outlet pipe 34 connected to theoutside.

The exhaust gas discharged from the internal combustion such as engineor the like is introduced through the inlet pipe 33 into the apparatus30 for the purification of the exhaust gas and then flowed through thecells opened to the inlet side (the outlet side is sealed with theplugging material 22 into the filter F for the purification of theexhaust gas and passed through the cell walls to catch particulates onthe cell walls 23, and the purified gas is discharged from the cellsopened to the outlet side toward the outside of the apparatus F for thepurification of the exhaust gas and further through the outlet pipe 34to the outside. Moreover, if it is intended to convert gas components ofnon-particular form, the plugging material 22 is not required.

As a greater amount of particulates are deposited on the cell walls 23of the filter F for the purification of the exhaust gas in the exhaustgas purifying apparatus 30 to increase the pressure loss, the filter Ffor the purification of the exhaust gas is subjected to a reproductiontreatment. In this reproduction treatment, a gas heated by using aheating means such as exhaust gas, catalyst disposed if necessary,heater or the like is introduced into the insides of the cells 21 in thefilter F for the purification of the exhaust gas to heat the filter Ffor the purification of the exhaust gas, whereby the particulatesdeposited on the cell walls 23 are burnt and removed.

Recently, the environment surrounding the above honeycomb structure forthe purification of the exhaust gas is largely changed, so that the sealmaterial as a constitutional element of the structure is required tohave higher sealing property and adhesion strength, but the honeycombstructure itself is demanded to more improve the durability over a longperiod of time.

As mentioned above, the honeycomb structure and seal material accordingto the invention can expect effects that when the layer of the sealmaterial having the construction inherent to the invention is arrangedin the space between the mutual honeycomb units or disposed on the outerperipheral portion of the honeycomb block formed by binding theplurality of the honeycomb units, since the particle size of the oxideparticles in the seal material layer is about 0.01 to about 100 μm,cracks and the like through thermal shock hardly occurs and the highsealing property and adhesiveness are developed, and also when it isused under an environment easily subjected to thermal shock as in afilter for the purification of the exhaust gas or the like, thedurability can be kept over a long time of period and further it is welldurable to the reproduction treatment by heating to improve the servicelife.

Next, an example of the production method of the honeycomb structure 10according to the invention will be described.

At first, a green shaped body of a honeycomb unit is produced bypreparing a starting paste composed mainly of the above startingmaterials (one kind of material in case of the usual honeycomb unit, orinorganic material as a main material and inorganic material as asub-material and inorganic binder or the like in case of the multi-mixedtype honeycomb units) and subjecting the paste to an extrusion shapingor the like. The starting paste may be properly added with an organicbinder, a dispersion medium and a shaping assist in addition to theabove materials. As the organic binder can be used one or more organicbinders selected from methylcellulose, carboxymethyl cellulose,hydroxyethyl cellulose polyethylene glycol, phenolic resin and epoxyresin. The amount of the organic binder compounded is preferable to beabout 1 to about 10% by weight based on 100 parts by weight of a totalof the inorganic material of the first form, the inorganic material ofthe second form and the inorganic binder. As the dispersion medium canbe used water, an organic solvent (benzene and the like) and an alcohol(methanol and the like). As the shaping assistant can be used ethyleneglycol, dextrin, aliphatic acid, fatty acid, fatty acid soap andpolyalcohol.

The starting paste is preferable to be well mixed and milled, so that itmay be sufficiently milled by using a mixer, an attritor, a kneader orthe like. In the shaping of the starting paste, it is preferable thatthe cells, and the cell walls are formed, for example, by extrusionshaping or the like.

Then, the green shaped body is dried by using a drying machine such as amicrowave drier, a hot air drier, a dielectric drier, a drier under areduced pressure, a vacuum drier, a freezing drier or the like. Ifnecessary, the drying is conducted after either one of end portions ofthe cells is sealed with a seal material.

Next, the green shaped body is degreased. The degreasing conditions areproperly adjusted in accordance with the kind and amount of the organicbinder included in the green shaped body, but it is preferable toconduct the degreasing at about 400° C. for about 2 hours. Further, thedried and degreased shaped body is fired. The firing is preferable to becarried out at a temperature of about 600 to about 2200° C.Particularly, it is preferably to be about 600 to about 1200° C. in caseof the oxide ceramic, and it is preferable an inert atmosphere of about1000 to about 2200° C. in case of the nitride and carbide ceramics.Through these steps can be obtained a honeycomb unit made from a porousceramic member of a honeycomb structure having plural cells.

Thereafter, the honeycomb unit is fired, if necessary, and subjected toa heat treatment in an oxidizing atmosphere above about 700° C., wherebyan oxide film is formed on the surface of the honeycomb unit, or thecoating of an oxide ceramic is carried out to form an oxide layer.

If necessary, it is possible to adjust a surface roughness by subjectingthe surface of the honeycomb unit to a working with sand blast or thelike.

Then, a seal material paste made of the above starting material isapplied onto the outer peripheral surface of the thus obtained honeycombunit to pre-join a plurality of honeycomb units (16 units in FIG. 1).Moreover, the seal material paste in the following examples is used byadding the aforementioned inorganic binder with the aforementionedinorganic particles, inorganic fiber and organic binder.

Next, the pre-joined honeycomb units are dried and fixed to obtain ahoneycomb block (honeycomb structure) as a joined body of the honeycombunits having a given size. In this case, the drying temperature somewhatchanges in accordance with the kind and amount of the organic substance,but is usually a range of about 100 to about 200° C.

Moreover, the seal material layer 11 interposed between the mutualhoneycomb units may be a dense body or may be a porous body capable offlowing the exhaust gas. However, the seal material layer 12 as acoating material layer for the outermost layer is desirable to be atleast made of a dense body. The seal material layer 12 is used for thepurpose of preventing the leakage of the exhaust gas from the outerperiphery of the honeycomb block when the assembly type honeycombstructure of the invention is disposed in the exhaust path of theinternal combustion.

The seal material layer 11 used for joining the honeycomb units to eachother is preferable to have a thickness of about 0.5 to about 3 mm. Whenthe thickness of the seal material layer 11 is about 0.5 mm or more, thesufficient adhesion strength is easily obtained, while when thethickness of the seal material layer is about 3 mm or less, the pressureloss may be hardly increased.

The number of the honeycomb units joined may be properly decided inaccordance with the size of the honeycomb structure. Also, the honeycombblock (joint body) formed by adhering the porous honeycomb units withthe seal material layer is properly subjected to a finish treatment suchas cutting, polishing or the like.

Moreover, when the form of the honeycomb block is made to a form of thepredetermined honeycomb structure (e.g. cylindrical shape) formed byshaping porous honeycomb units of fan-shaped or square section, it ispossible to omit the cutting or polishing step of the honeycomb block.

For the purpose of coating the outer peripheral surface of the honeycombstructure, i.e. the side face not opening the through-holes (cellholes), the seal material is applied and fixed by drying to form theseal material layer 12. The coating material layer 12 is preferablyexistent for protecting the outer peripheral surface of the honeycombunit to enhance the strength. In this case, the seal material is notparticularly limited, but may be the same as the seal material usedbetween the honeycomb units or a different material. The coatingmaterial may have the same compounding ratio as in the aforementionedseal material or a different compounding ratio. The coating materiallayer is preferable to have a thickness of about 0.1 to about 3 mm. Whenthe thickness is 0.1 mm or more, the outer peripheral surface issufficiently protected and the leakage of the gas hardly occurs and thestrength can be easily enhanced. While, when it is about 3 mm or less,if thermal stress or the like is applied to the honeycomb structure,cracks hardly occur or the pressure loss hardly increases. The dryingand fixation of the coating layer may be carried out under substantiallythe same conditions as in the aforementioned seal material layer.

Further, the plurality of the honeycomb units are joined through theseal material and then calcined. But, when a coating material layer isapplied to the outer peripheral surface thereof, a calcination iscarried out after applying the coating material layer. If the organicbinder is included in the seal material and the coating material, thedegreasing can be conducted by the calcination. The calcinationconditions are properly determined in accordance with the kind andamount of the organic substance included, but it is preferable toconduct the calcination at about 400 to about 800° C. for about 1 toabout 2 hours. In case of using the thus calcined honeycomb structure,there is caused no discharge of the contaminated gas due to the burningof the organic binder remained in the honeycomb structure.

The application of the thus obtained honeycomb structure according tothe invention is not particularly limited, but it can be used as acatalyst carrier for the conversion of the exhaust gas in theautomobiles or a diesel-particulate-filter having a function offiltering particulate substance in the exhaust gas to purifying byburning.

Also, a honeycomb catalyst may be formed by supporting a catalystcomponent on the thus obtained honeycomb structure. The catalystcomponent is not particularly limited, but may be a noble metal, analkali metal compound, an alkaline metal compound, an oxide or the like.As the noble metal are mentioned one or more selected from platinum,palladium and rhodium. As the alkali metal compound are mentionedcompounds of one or more selected from potassium, sodium and the like.As the alkaline metal compound are mentioned of compounds of barium andthe like. As the oxide are mentioned perovskite (La_(0.75)K_(0.25)MnO₃and the like), CeO₂ and the like. The resulting honeycomb catalyst isnot particularly limited, but can be used as a so-called three-waycatalyst for the purification and/or conversion of the exhaust gas inthe automobile or NOx absorption catalyst.

EXAMPLES

The following examples are given in illustration of the invention andare not intended as limitations thereof.

(Test 1)

This test is carried out for confirming the action and effect of each ofplural seal material layers (adhesive material, coating material)prepared by variously changing materials such as oxide and the like andformed on an outer surface of a honeycomb unit made from siliconcarbides having various surface roughnesses or a fiber-reinforcedalumina.

(Preparation of a Paste for Seal Material)

Compounding recipes of pastes 1-27 are shown in Table 1.

As an example of a paste for the seal material, a heat-resistant pastefor the seal material is first formed by mixing 30% by mass of inorganicpowder (a-alumina particles, average particle size: 0.01 μm), 10% bymass of inorganic fibers (silica-alumina fiber, average fiber diameter:10 μm, average fiber length: 200 μm), 30% by mass of silica sol (solidcontent: 30% by mass), 5% by mass of carboxymethyl cellulose and 25% bymass of water. This is a paste 1. Moreover, α-alumina is confirmed to becrystalline because a peak is observed by an X-ray diffractometry. Thiscrystalline alumina is used in all of the following materials.

Similarly, 27 kinds of the pastes are prepared by changing thecompounding ratio of the starting powder, inorganic fiber, silica soland carboxymethyl cellulose as shown in Table 1.

Then, 10 g of each of the pastes is sandwiched between two plates at athickness of 2 mm and dried at 110° C. for 1 hour. As a result, thepastes 25, 26, 27 show the lowering of the plasticity and generate gapsin places between the plates as shown in Table 1. Therefore, the pastes25, 26 and 27 are not used in the following examples.

Table 1

TABLE 1 Inorganic Inorganic Silica-alumina silica sol powder: powder:Inorganic Average particle size fiber(mass %) (mass %) Carboxymethylαalumina Zirconia powder: of inorganic powder fiber length 200 μm solidcontent cellulose Water (mass %) (mass %) SiC(mass %) (μm) fiberdiameter 10 μm 30mass % (mass %) (mass %) Gaps Paste 1 30 0 0 0.01 10 305 25 none Paste 2 30 0 0 0.1 10 30 5 25 none Paste 3 30 0 0 0.5 10 30 525 none Paste 4 30 0 0 10 10 30 5 25 none Paste 5 30 0 0 75 10 30 5 25none Paste 6 30 0 0 100 10 30 5 25 none Paste 7 30 0 0 200 10 30 5 25none Paste 8 30 0 0 0.005 10 30 5 25 none Paste 9 0 30 0 0.01 10 30 5 25none Paste 10 0 30 0 0.1 10 30 5 25 none Paste 11 0 30 0 0.5 10 30 5 25none Paste 12 0 30 0 10 10 30 5 25 none Paste 13 0 30 0 75 10 30 5 25none Paste 14 0 30 0 100 10 30 5 25 none Paste 15 0 30 0 200 10 30 5 25none Paste 16 0 30 0 0.005 10 30 5 25 none Paste 17 0 0 30 0.01 10 30 525 none Paste 18 0 0 30 0.1 10 30 5 25 none Paste 19 0 0 30 0.5 10 30 525 none Paste 20 0 0 30 10 10 30 5 25 none Paste 21 0 0 30 75 10 30 5 25none Paste 22 0 0 30 100 10 30 5 25 none Paste 23 0 0 30 200 10 30 5 25none Paste 24 0 0 30 0.005 10 30 5 25 none Paste 25 30 0 0 300 10 30 525 presence Paste 26 0 30 0 300 10 30 5 25 presence Paste 27 0 0 30 30010 30 5 25 presence

(Preparation of Honeycomb Structure)

The honeycomb unit is prepared by the following two methods.

(Preparation of Silicon Carbide Honeycomb Unit)

A starting material is prepared by mixing 80% by mass of silicon carbidepowder having an average particle size of 8.5 μm and 20% by mass ofsilicon carbide powder having an average particle size of 0.2 μm.

Then, 10 parts by mass of methyl cellulose is added and mixed withrespect to 100 parts by mass of the starting powder. Also, 18 parts bymass of a dispersion medium comprising of an organic solvent and wateris added to knead all of the materials. Finally, it is extrusion-shapedin a mold into a target honeycomb form to obtain a honeycomb shaped bodyhaving many through-holes (cell holes) and then either one end portionsof these through-holes are sealed in a checkered form to produce ahoneycomb shaped body. The honeycomb shaped body is dried at 150° C.,degreased at 500° C. and fired at 2200° C. in an inert gas atmosphere toobtain a honeycomb unit of 34.3 mm×34.3 mm×150 mm.

Next, the honeycomb unit is subjected to a heat treatment in air ofabout 1000° C. under conditions shown in Table 5 (heating time), ifnecessary to produce an oxide film on the surface of the unit.

Moreover, the thickness of the oxide film is an analytical value in adepth direction through AES. An analytical apparatus is JAMP-7800F (madeby Nippon Denshi Co., Ltd.), and the measuring conditions are an appliedvoltage of 15 kV, an irradiation current of 5×10̂(−8) A, and a beamdiameter of about 100 nmφ, and the etching conditions are an ion kind ofAr̂(+) and an etching grade of 2.8 nm/min as SiO₂ conversion. Theanalytical value is an average value at three places after the etching.

(Preparation of Fiber-Reinforced Alumina Honeycomb Unit)

At first, 40% by weight of γ-alumina particles (average particle size: 2μm), 10% by weight of silica-alumina fibers (average fiber diameter: 10μm, average fiber length: 100 μm, aspect ratio: 10) and 50% by weight ofsilica sol (solid content: 30% by weight) are mixed and 6 parts byweight of methyl cellulose as an organic binder and small amounts of aplasticizer and a lubricant are added on 100 parts by weight of theresulting mixture, which are further mixed and kneaded to obtain a mixedcomposition. Then, the mixed composition is shaped through an extrusionmachine to obtain a green shaped body.

The green shaped body is sufficiently dried by using a microwave drierand a hot drier and degreased at 400° C. for 2 hours. Thereafter, it isfired at 800° C. for 2 hours to obtain a sample of a porous aluminaceramic honeycomb unit having a square pillar shaped (34.3 mm×34.3mm×150 mm), a cell density of 93 cells/cm² (600 cpsi) and a rectangularcell form (square).

(Preparation of Honeycomb Structure)

Samples of honeycomb units having various surface roughness of Ra −0.5μm, 1.0 μm, 5.0 μm, 15 μm, 20 μm, 30 μm and 40 μm are obtained bysubjecting an outer surface of each of the honeycomb units to a workingtreatment with sand blast having different abrasion grain percentages.

In this case, a value of surface roughness Ra on an outer wall of eachsample is measured by means of a surface roughness measuring machine(made by Tokyo Seimitsu Co., Ltd. SURFCOM 920A) by scanning a centralportion of the outer surface (four sides) of each sample (honeycombunit) so as to be parallel to the cell, whereby numerical values at 4outer surfaces of the honeycomb unit are matched with each other. Afterthe adjustment of the surface roughness, the formation of the aboveoxide film is conducted to provide example samples.

Then, there are provided 16 units of each of the samples. The unitshaving the same level are adhered to each other with the above paste forthe seal material (Table 1, Nos. 1-24) by drying at 150° C. for 2 hoursand firing at 500° C., and thereafter the outer peripheral portionthereof cut by a diamond cutter to obtain a columnar ceramic block(assembly type honeycomb structure).

In this case, the seal material layer is 2 mm. Further, the same pastefor the seal material is applied onto the outer peripheral portion ofthe ceramic block to form a seal material layer (coating material layer)of 2 mm, whereby there is produced a honeycomb filter for thepurification of the exhaust gas (diameter: 144 mm, length: 150 mm).

Moreover, the silicon carbide honeycomb units are joined with thealumina-based seal material (pastes 1-6, and as a comparative example,pastes 7, 8, 17-24), and the fiber-reinforced alumina honeycomb unitsare joined with a zirconia-based seal material (pastes 9-14, and as acomparative example, pastes 15-24).

(Evaluation Test)

(1) Thermal Shock Test (Outer Peripheral Portion of Seal Material Layer)

The honeycomb structure is placed in an electric furnace and subjectedto a thermal shock test by heating at a temperature rising rate of 5°C./min and keeping at a temperature of 700° C. for 30 minutes and slowlycooling to room temperature (20° C.). The results of the thermal shocktest are shown in Tables 2 and 3.

As a result, the pastes 1-6 and 9-14 using the seal material in whichthe average particle size of alumina or zirconia in the seal material is0.01 μm-100 μm adaptable to the invention do not show the occurrence ofcracks even when the surface roughness Ra of the honeycomb unit is0.1-40 μm, while when the particle size is 0.005 μm or 200 μm, cracksare generated in the honeycomb structure.

Table 2

TABLE 2 <Cracked in outer> Honyecomb unit Surface Seal material: averageparticle size of alumina particle (μm) roughness Paste 8 Paste 1 Paste 2Paste 3 Paste 4 Paste 5 Paste 6 Paste 7 Ra (μm) 0.005 0.01 0.1 0.5 10 75100 200 0.1 cracked cracked cracked cracked cracked cracked crackedcracked presence none none none none none none presence 1 crackedcracked cracked cracked cracked cracked cracked cracked presence nonenone none none none none presence 10 cracked cracked cracked crackedcracked cracked cracked cracked presence none none none none none nonepresence 15 cracked cracked cracked cracked cracked cracked crackedcracked presence none none none none none none presence 20 crackedcracked cracked cracked cracked cracked cracked cracked presence nonenone none none none none presence 30 cracked cracked cracked crackedcracked cracked cracked cracked presence none none none none none nonepresence 40 cracked cracked cracked cracked cracked cracked crackedcracked presence none none none none none none presence

Table 3

TABLE 3 <Cracked in outer> Honeycomb unit Surface Seal material: averageparticle size of zirconia particle (μm) roughness Paste 16 Paste 9 Paste10 Paste 11 Paste 12 Paste 13 Paste 14 Paste 15 Ra (μm) 0.005 0.01 0.10.5 10 75 100 200 0.1 cracked cracked cracked cracked cracked crackedcracked cracked presence none none none none none none presence 1cracked cracked cracked cracked cracked cracked cracked cracked presencenone none none none none none presence 10 cracked cracked crackedcracked cracked cracked cracked cracked presence none none none nonenone none presence 15 cracked cracked cracked cracked cracked crackedcracked cracked presence none none none none none none presence 20cracked cracked cracked cracked cracked cracked cracked cracked presencenone none none none none none presence 30 cracked cracked crackedcracked cracked cracked cracked cracked presence none none none nonenone none presence 40 cracked cracked cracked cracked cracked crackedcracked cracked presence none none none none none none presence

(2) Adhesion Test of Honeycomb Structure

After the honeycomb structure is disposed on a hollow cylindrical jig,one honeycomb filter is selected from the honeycomb structure atsubstantially a central zone thereof and then a pressure in a pushingdirection is applied to this honeycomb filter through a stainlesscylindrical jig having a diameter of 31 mm to measure a load till thebreakage (adhesion strength), which is a pushing load in the jointportion of the seal material layer (breakage load). The results areshown in Tables 4-6.

Table 4 shows the pushing load when the alumina particle size in theseal material and the surface roughness of the honeycomb unit arechanged in the honeycomb structure formed by adhering the siliconcarbide honeycomb units with the alumina seal material, which are shownin FIGS. 4 and 5. As seen from these results, when the particle size is0.01-100 μm, the high pushing load is obtained.

Table 5 shows the pushing load when the alumina particle size in theseal material and the oxide film in the honeycomb unit are changed inthe honeycomb structure formed by adhering the silicon carbide honeycombunits with the alumina seal material, which are shown in FIGS. 6 and 7.In case of using the seal material having an alumina particle size of0.01-100 μm, the high pushing load is obtained.

Table 6 shows the pushing load when the zirconia particle size in theseal material and the surface roughness of the honeycomb unit arechanged in the honeycomb structure formed by adhering thefiber-reinforced alumina honeycomb units with the alumina seal material,which are shown in FIGS. 8 and 9. As seen from these results, in case ofthe seal material having a zirconia particle size of 0.01-100 μm, thehigh pushing load of from 640 to 994 kg is obtained.

Table 4

TABLE 4 <Pushing load (kg)> Honeycomb unit Surface Seal material:average particle size of alumina particle (μm) roughness Paste 8 Paste 1Paste 2 Paste 3 Paste 4 Paste 5 Paste 6 Paste 7 Ra (μm) 0.005 0.01 0.10.5 10 75 100 200 0.1 380 700 735 850 738 660 460 350 1 470 770 820 920820 740 550 450 10 540 860 897 1010 898 823 635 520 15 570 870 919 1025920 840 650 550 20 524 850 883 1000 885 810 613 505 30 480 774 821 930823 744 555 458 40 390 705 743 860 745 674 467 365 Honeycomb unitSurface Seal material: average particle size of SiC particel (μm)roughness Paste 24 Paste 17 Paste 18 Paste 19 Paste 20 Paste 21 Paste 22Paste 23 Ra (μm) 0.005 0.01 0.1 0.5 10 75 100 200 SiC: 0.1 μm 380 380380 382 380 380 380 380

Table 5

TABLE 5 <Pushing load (kg)> Oxide layer thickness of Seal material:average particle size of alumina particle (μm) Heating honeycomb Paste 8Paste 1 Paste 2 Paste 3 Paste 4 Paste 5 Paste 6 Paste 7 time unit (nm)0.005 0.01 0.1 0.5 10 75 100 200 (minutes) 0.5 380 700 735 850 738 660460 350 1 1 470 770 820 920 820 740 550 450 10 50 540 860 897 1010 898823 635 520 100 100 570 870 919 1025 920 840 650 550 150 500 524 850 8831000 885 810 613 505 200 1000 480 774 821 930 823 744 555 458 300 2000390 705 743 860 745 674 467 365 400 Oxide layer thickness of Sealmaterial: average particle size of SiC particle (μm) Heating honeycombPaste 24 Paste 17 Paste 18 Paste 19 Paste 20 Paste 21 Paste 22 Paste 23time unit (nm) 0.005 0.01 0.1 0.5 10 75 100 200 (minutes) SiC: 1 nm 380380 380 382 380 380 380 380 1

Table 6

TABLE 6 <Pushing load (kg)> Honeycomb unit Surface Seal material:average particle size of zirconia particle (μm) roughness Paste 16 Paste9 Paste 10 Paste 11 Paste 12 Paste 13 Paste 14 Paste 15 Ra (μm) 0.0050.01 0.1 0.5 10 75 100 200 0.1 368 679 714 824 715 640 446 339 1 455 746796 892 795 717 533 436 10 523 834 872 980 871 798 615 504 15 552 843890 994 892 814 630 533 20 508 824 855 970 858 785 594 489 30 465 751797 902 798 721 538 444 40 378 683 722 834 722 653 452 354 Honeycombunit Surface Seal material: average particle size of SiC particle (μm)roughness Paste 24 Paste 17 Paste 18 Paste 19 Paste 20 Paste 21 Paste 22Paste 23 Ra (μm) 0.005 0.01 0.1 0.5 10 75 100 200 SiC: 0.1 μm 380 380380 382 380 380 380 380

In general, it is said that a load of about 1.0-2.0 kg/cm² (98.0−196.1kPa) per unit area is applied to the honeycomb structure for thepurification of the exhaust gas in the automobile by vibrations of theengine and pressure of the exhaust gas in case of the filter of 34.3mm×34. 3 mm×150 cm. When this load is applied to the test specimen(sample), the adhesion strength required for the seal material layer(load supporting area: 206 cm₂) joining one honeycomb unit (34.3 mm×34.3 mm×150 mm) in the honeycomb structure is about 412 kg at maximum. Fromthis result, it is clear that the seal material layer shows theadhesiveness capable of sufficiently putting into practical use.

As seen from FIGS. 4 and 8, when the surface roughness Ra of the outersurface of the honeycomb unit or honeycomb monolith is 1-30 μm, thesealing property is good. Also, as shown in FIG. 6, the sealing propertyis more improved when the oxide film is existent.

As to the pastes 1-14 for the seal material, each sample is shaped intoa disc having a thickness of 5 mm and a diameter of 5 cm, which is driedand heat-treated. Then, the density of the sample is measured and thethermal conductivity is measured by a laser flash method. The resultsare shown in Tables 7 and 8.

Table 7

TABLE 7 Seal material: average particle size of alumina particle (μm)Density Paste 8 Paste 1 Paste 2 Paste 3 Paste 4 Paste 5 Paste 6 Paste 7kg/m³ = 2000 0.005 0.01 0.1 0.5 10 75 100 200 Thermal 0.34 0.35 0.450.596 0.5 0.4 0.35 0.3 conductivity λ(W/mk)

Table 8

TABLE 8 Seal Material: average particle size of zirconia particle (μm)Density Paste 16 Paste 9 Paste 10 Paste 11 Paste 12 Paste 13 Paste 14Paste 15 kg/m³ = 2200 0.005 0.01 0.1 0.5 10 75 100 200 Thermal 0.3 0.330.43 0.48 0.432 0.40 0.3 0.2 conductivity λ(W/mk)

As seen from the results, when the alumina particle size or zirconiaparticle size of the seal material is 0.1 to about 75 μm, particularly0.1 to 10 μm, the high thermal conductivity is obtained. This is guesseddue to the fact that the filling property is improved to increase thecontact places while gaps between particles are generated in case ofcoarse particles and second particles are produced in case of finerparticles to generate gaps between the particles.

This application claims a benefit of priority based on PCT/JP2005/006532filed on Mar. 28, 2005, the entire contents of which patent applicationis hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

The invention is used as a filter in not only an apparatus for thepurification and/or conversion of the exhaust gas in the internalcombustion but also an apparatus for purifying and/or converting theexhaust gas discharged from a boiler, a heating furnace, a gas turbineor various industrial processes. Particularly, it is useful as acatalyst carrier for the convertion of the exhaust gas in theautomobiles or a diesel-particulate-filter (DPF) having a function offiltering and combustion-purifying particulate matter (PM) in theexhaust gas. Of course, it can be utilized in applications withoutsupporting the catalyst component (e.g. adsorbent or the like adsorbinggas components or liquid components) and the like.

1-19. (canceled)
 20. A honeycomb structure, comprising: an assemblyhaving a plurality of honeycomb units, each of the plurality ofhoneycomb units comprising: inorganic ceramic particles; inorganicreinforcing material for enhancing the strength of the honeycomb unit; aplurality of through-holes and two end faces, each at one of the twoopposite ends of the honeycomb unit, wherein the through-holes extendfrom one end face to the opposing end face; and a layer of seal materialinterposed between and in direct contact with abutting honeycomb unitsto adhere the plurality of honeycomb units together, wherein the sealmaterial comprises an inorganic binder and oxide particles having aparticle size of about 0.01 μm to about 100 μm.
 21. A honeycombstructure according to claim 20, wherein the inorganic reinforcingmaterial comprises one or more kinds of inorganic fibers or whiskers.22. A honeycomb structure according to claim 21, wherein the inorganicfibers are selected from a group consisting of alumina fibers, silicafibers, silicon carbide fibers, silica-alumina fibers, glass fibers,potassium titanate fibers, and aluminum sulfate fibers.
 23. A honeycombstructure according to claim 20, wherein the honeycomb unit has asectional area at a cross section perpendicular to the longitudinaldirection of the honeycomb unit of about 5 to about 50 cm².
 24. Ahoneycomb structure according to claim 20, wherein a layer of sealmaterial envelops a peripheral portion of the assembly between theopposing end faces of the honeycomb units.
 25. A honeycomb structureaccording to claim 20, wherein the oxide particles in the seal materiallayer comprise one or more kinds of particles selected from the groupconsisting of alumina, zirconia, titania, and silica.
 26. A honeycombstructure according to claim 20, wherein the oxide particles have aparticle size of about 0.1 μm to about 75 μm.
 27. A honeycomb structureaccording to claim 20, wherein the honeycomb unit comprises a ceramicoxide.
 28. A honeycomb structure according claim 20, wherein a surfaceof the honeycomb unit in contact with the seal material layer has aroughness Ra of about 1.0 to about 30 μm.
 29. A honeycomb structureaccording to claims 20, wherein the honeycomb unit has a catalystsupported on a wall surface of the through-hole.
 30. A honeycombstructure according to of claims 20, said honeycomb structure is used asan apparatus for the purification of an exhaust gas in an automobile.